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Are AUSTROADS Pavement Design Performance Models Adequately Calibrated for New Zealand?
Dr Bryan PidwerbeskyGeneral Manager - TechnicalFulton Hogan Ltd
AUSTROADS PTF Workshop, Wellington 04 December 2014
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Outline
• Asphalt fatigue strain criterion• Subgrade strain criterion• Terminal rut depth• Back-calculation from FWD deflection bowls• Conclusion/summary
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AUSTROADS Pavement Design Guide
1 Horizontal tensile strain in bottom of asphalt – fatigue cracking2 Horizontal tensile strain in bottom of cemented material - cracking3 Vertical compressive strain in top of subgrade - rutting & shape loss
Subgrade 3
Asphalt 1
Cemented Material 2
Unbound subbase
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• Inadequate load supporting capacity:– Loss of base, subbase or subgrade support (eg
water ingress) → high deflection and/or deformation
– Inadequate thickness of the pavement to take the loads
– Increase in loading – Poor construction
Causes of Cracking in Asphalt
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• Reflective cracking (from underlying asphalt, stabilised base or subgrade)
Causes of Cracking in Asphalt
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• Brittle Failures– Old oxidized asphalt– Asphalt too stiff for environmental conditions
Outside Wheel Path
Little Shape loss
Causes of Cracking in Asphalt
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• Thermal-induced cracking
Causes of Cracking in Asphalt
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Causes of Cracking in Asphalt
• Classic fatigue-induced cracking is rare in New Zealand
• Cracks normally start at top of asphalt– Start as very fine cracks created during
roller compaction– Largest tensile strain is at top of asphalt– rarely bottom up
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• Fatigue strainVery small strains (~100 με) per loading
• Flexure strainLarger strains exceed maximum tensile strain capacity
• Thermal-induced strainEnvironmental factors
Causes of Cracking in Asphalt
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The History of Asphalt Fatigue Criterion
Asphalt fatigue criterion
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The History of Asphalt Fatigue Criterion
Pell, P.S. (1962) Fatigue Characteristics of Bitumen and Bituminous Mixes. Int’l Conference on Structural Design of Asphalt Pavements, Ann Arbor, USA.
Asphalt fatigue criterion
Fatigue lives for different mixes at 0°C showing derived bitumen strain
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Asphalt Fatigue Relationship
• 1960’s - Laboratory-derived fatigue relationship• 1970’s - Adjusted to predict fatigue life in pavements using
a shift factor F
• N = allowable number of load repetitions• µε = tensile microstrain produced by the load
• VB = % by volume of binder in asphalt
• Smix =mix stiffness modulus (MPa)
• F = range of values
5
36.0B
1.08) V 6918(0.856 F
mixSN
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Shift Factors
• Shell Pavement Design Manual (1978) : F=10• Saunders, L.R. A Modern Basis for Pavement Design (1982) : F = 10• AUSTROADS Pavement Design Guide (1992)
Ignored shift factor (F = 10 was considered)• Baburamani, ARR 334 Asphalt Fatigue Life Predictions Models (1999)
F = 10 to 20• AUSTROADS Pavement Design Guide (2001 draft) : F = 5• Saleh (2012) : F = 5.7
5
36.0B
1.08) V 6918(0.856 F
mixSN
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AUSTROADS Guide (2014) Table 6.15Suggested Reliability Factors for Asphalt Fatigue
Desired project reliability
80% 85% 90% 95% 97.5%
RF 2.5 2.0 1.5 1.0 0.67
Desired project reliability has two components: • a shift factor relating mean laboratory fatigue life to a mean in-service fatigue
life, taking account of differences between laboratory test conditions and conditions applying to in-service pavement;
• a reliability factor relating mean in-service fatigue life to in-service predicted life at a desired project reliability, taking into account factors such as construction variability, environment and traffic loading
• “for lightly-trafficked roads load-induced fatigue cracking is uncommon.”
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Reliability factor/shift factor is too low
• Confusion about what constitutes fatigue cracking
• Fatigue cracking is result of millions of very small resilient strains under wheel loadings, at significantly less than horizontal strain capacity of bound material
• In majority of cases, crack-induced failures are actually due to excess deflection/flexure of the underlying pavement &/or subgrade, causing significant tensile strain in asphalt that exceeds its tensile strain capacity
• Fatigue criterion not applicable to thin asphalt surfacings
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Appropriate shift/reliability factor
• Saunders (1982) 10• Saleh (2012) 5.7• Experience 5-10
Recommended Reliability Factors Desired project reliability
80% 85% 90% 95% 97.5%
RF 10 5 4 3 2.5
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“...the primary function of a road structure is to protect the underlying soil from excessive stresses produced by traffic loads....”“It is therefore necessary to limit the deformation in the soil and this may be done by limiting the value of the vertical compressive stress reaching the top of the subgrade....”“… the value of the vertical stress in the subgrade is one of the critical quantities determining the performance of a flexible pavement.” Peattie, K.R. (1962) A Fundamental Approach to the Design of Flexible Pavements. Proc. Int’l Conference on the Structural Design of Asphalt Pavements, Ann Arbor
Subgrade strain criterion
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“Deformations of the surface under the action of repeated loadings by traffic is controlled by limiting the vertical compressive stress or strain in the subgrade, and if necessary on the other granular layers in the structure.”“…irrespective of the construction, the maximum vertical compressive strain in the top of the subgrade is 9 x 10-4, and for roads carrying greater traffic volumes, a permissible compressive strain should be 6.5 x 10-4.”
Dormon, G.M. (1962) The Extension to Practice of Fundamental Procedure for the Design of Flexible Pavements. Proc. Int’l Conference on the Structural Design of Asphalt Pavements, Univ. of Michigan, Ann Arbor.
Subgrade strain criterion
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11.4 11.9 12.4 12.9 13.40
20
40
60
80
100
120
140
160
Granular Overlay (mm) - Austroads (GMP-Rigorous) Granular Overlay (mm) - TNZ Precedent Method
Chainage (km)
Gra
nula
r Ove
rlay
(mm
)
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For unbound or stabilised granular pavements, subgrade strain criteria is conservativeActual measured strains are greater than permissible strains calculated according to the criteria.Vertical compressive strains in the basecourse can be as large (in magnitude) as vertical compressive strains in the subgrade
RecommendationStrains in the basecourse should be explicitly considered in the AUSTROADS pavement design procedure
Subgrade strain criterion
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Permanent subgrade strain/load is too small to measure
Subgrade strain criterion based on resilient subgrade strain because that is a much larger magnitude & can be measured
Assumed relationship between resilient & permanent subgrade strain
Accumulation of permanent subgrade strain manifests itself as pavement rutting
Thickness designs assume terminal rut depth is 20-25 mm
Terminal rut depth
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“Implicit in the design procedure for these pavements (Section 8.3 and, specifically, Figure 8.4 of the Guide) is a terminal condition which is considered to be unacceptable and, hence, signifies the end of life for the pavement.” “The view of the MEC Review Committee at the time was that, in terms of rutting, it represented an average rut depth of about 20 mm.”
AUSTROADS (2004) Technical Basis of AUSTROADS Pavement Design Guide. AP-T33/04
Terminal rut depth
Severity Level Rut (mm)Low 6 – 12.5
Moderate 12.5 - 25High >25 mm Typical Definitions of Rutting
(FHWA, 2011)
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0 2 4 6 8 10 12 14 16 18 20 22 240
5
10
15
20
25
30Actual 1
Extrapolate 1
Actual 2
Extrapolate 2
Terminal Rut
Mainte-nance In-tervention
Terminal rut depth
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Deflection & Back-calculation
Back-calculation techniques based on FWD deflection bowls inaccurate for estimating pavement & subgrade properties:• Transfer functions are based on regression analyses & are
never calibrated for specific projects• Transposition of independent & dependent variables• CBR’s derived from back calculation only intended to be
relative & approximate, & used only in the context of pavement design overlays
• Derived CBR value is only for modeling requirements & cannot accurately reflect actual subgrade CBR - it has to be measured in lab or inferred from in situ tests
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Deflection & Back-calculation
Subgrade Bearing Capacity Rehabilitation ProjectParameter A B CCBR inferred from in-situ Scala Penetrometer 4% 4-5% 4%
Isotropic Modulus Backcalculated 69 MPa 35 MPa 86 MPa
Anisotropic Modulus Equivalent(1) 100 MPa 52 MPa 113 MPaLaboratory soaked Subgrade CBR 15% 25% Subgrade CBR assumed for design 5 4 5
(1) Modulus back-calculated from FWD deflection bowl: 10th percentile isotropic subgrade stiffness converted to practical equivalent anisotropic stiffness (EISO=0.67xEANISO(vert)) (Tonkin & Taylor, 1998)
Example data from actual projects shows variability in subgrade CBR values derived from different techniques
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Subgrade strain criterion was only ever intended to be used for design purposes & provides reasonable values, given cumulative effect of assumptions made during design process
Predictions of material properties and remaining life from back-calculation procedures (based on FWD deflection bowls) poorly correlated with actual performance
RecommendationTo use back-calculation procedures based on FWD deflections for estimating remaining life of a specific pavement contractually, models & algorithms used in procedure must be robustly validated for specific conditions of each site
Deflection & Back-calculation
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For fatigue cracking in bitumen-bound layers, project reliability factors should be in range of 2.5 to 5 (at least) for New Zealand
Asphalt fatigue criterion is not applicable to thin surfacings
Vertical compressive & shear strains within unbound & modified pavement layers should be explicitly considered as a critical parameter in flexible pavement design
Terminal rut depth for unbound granular/ stabilised flexible pavements is 20 mm
Back-calculation procedures based on FWD deflection data may be used to estimate remaining life of a pavement ONLY after models & algorithms have been robustly validated for specific conditions of each site
Conclusion/Summary
